In fourth generation mobile telecommunications technology networks, i.e., 4G networks, real time services such as audio and/or video are provided over Internet Protocol (IP) using sessions. For example, multimedia services can and sometimes are provided over Internet Protocol Multimedia Core Network Subsystems. However, 4G network coverage is not universal and there are currently many geographical areas with third generation mobile telecommunications technology network coverage, i.e., 3G coverage, but no 4G coverage. When a device moves from a geographic location having 4G coverage to a geographic location having 3G coverage, the device is assigned a new IP address during what is sometimes referred to as a 4G to 3G fallback operation. This causes a change in the media IP Address used by the device for on-going sessions. One known approach of handling this change in the IP Address for on-going sessions requires that the new IP Address be communicated to the other end of the session. However, this approach introduces a non-negligible amount of delay during the transition from 4G to 3G. As a result, a new network element referred to as an Access Transfer Gateway (ATGW) which is controlled by an Access Transfer Control Function (ATCF) has been proposed. See the technical specification 3GPP TS 29.334 V10.2.0 produced by the 3rd Generation Partnership Project (3GPP) and the technical specification ETSI TS 123 237 V12.7.0 produced by the ETSI 3rd Generation Partnership Project. This network element is introduced to and remains on the media call path from the beginning of call. When a change in a device's IP address occurs during a session, the new IP address of the device is communicated only to the ATGW and the far end device is shielded from the change eliminating the latency introduced by sending a signaling message from the device with the changed IP address to the far end device.
Introducing an Access Transfer Gateway into the media path for all calls introduces a number of problems. First a new device is needed with a very high aggregate media handling capability and second latency is introduced to media streams for all calls as the media streams for all calls are routed through the Access Transfer Gateway.
In view of the above discussion, there is a need for new methods and apparatus for determining whether or not to include an Access Transfer Gateway in a call flow.